Oxidative dehydrogenation catalyst

ABSTRACT

METHOD FOR THE OXIDATIVE DEHYDROGENATION OF PARAFFINS TO PRODUCE OLEFINS BY CONTACTING THE PARAFFINS WITH MOLECULAR OXYGEN IN THE PRESENCE OF MOLTEN ALKALI METAL HYDROXIDES, ALUMINUM, AND A SOLUBLE TRANSITION METAL OXYANION INCLUDED IN THE MOLTEN ALKALI METAL HYDROXIDE.

United States Patent US. Cl. 252-467 Claims ABSTRACT OF THE DISCLOSUREMethod for the oxidative dehydrogenation of parafiins to produce olefinsby contacting the parafiins with molecular oxygen in the presence ofmolten alkali metal hydroxides, aluminum, and a soluble transition metaloxyanion included in the molten alkali metal hydroxide.

RELATED APPLICATIONS This application is a divisional application of US.Ser. No. 124,979, filed Mar. 16, 1971, and now US. Pat. 3,697,614.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to a novel catalytic method for the dehydrogenation ofparafiins.

Prior art US. Ser. No. 844,608, filed July 24, 1969, now US. Pat.3,586,733, discloses a method for the oxidative dehydrogenation ofparafiins to produce olefins by contacting the paraffins with molecularoxygen in the presence of molten alkali metal hydroxides, and aluminummetal or activated alumina.

SUMMARY OF THE INVENTION In accordance with this invention a gaseousmixture of a paraffin (alkane) having from 2 to 12 carbon atoms,preferably from 3 to 6 carbon atoms, together with molecular oxygen ispassed through a bed of molten alkali metal hydroxide containing asoluble transition metal oxyanion dissolved therein and aluminum metal,thereby dehydrogenating the parafiin of the corresponding olefin. It isan object of this invention to provide an improved method for theoxidative dehydrogenation of parafiins.

It is another object of this invention to provide a method for theoxidative dehydrogenation of paraflins to olefins utilizing molecularoxygen. It is a further object of this invention to provide a method forthe oxidative dehydrogenation of parafiins to olefins using molecularoxygen, molten alkali metal hydroxide, aluminum metal, and a transitionmetal oxyanion dissolved in the melt.

Other objects of this invention will become apparent from the followingdescription of the invention.

DETAILED DESCRIPTION OF THE INVENTION The paraffin (alkane) feed to theprocess of the inventionpreferably has from 2 to 12 carbon atoms and isstraight chain or branched chain, straight chain paraffins beingsomewhat more preferred. More preferably, the paraffin has from 3 to 6carbon atoms and the most preferred paraffin is propane.

The molecular oxygen is introduced along with the paraffin feed and ispreferably diluted with an inert gas such as nitrogen. It has been foundthat the optimum hydrocarbon to oxygen ratio is between about 1.8:1 and5.021. In general, the volume ratio of oxygen to nitrogen can vary fromabout 1:2 to 1:6 but preferably it is about 1:4 since this provides asafety factor relative to the exice plosive limits of thehydrocarbon-oxygen mixture. Air has been found to be a satisfactorysource of molecular oxygen. The paraffin and molecular oxygen arecontinuously passed through a reactor containing molten alkali metalhydroxide. The preferred alkali metal hydroxide is sodium hydroxide.

Aluminum metal is preferably included in the reactor, most preferably inthe molten alkali metal hydroxide melt, and is preferably in the shapeof small rings or irregular shapes in order to provide reaction surfacewhile at the same time providing a packed condition which al lows thefree passage of gases therethru.

The amount of aluminum need only be rather small, in general about 1 to5 grams of aluminum per ml. of the molten sodium hydroxide is suificient'but larger quantities can be employed within the scope of theinvention.

The reaction may be carried out at temperatures ranging from about 390C. to about 600 C. The preferred reaction temperature is between about425 C. and 500 C. The most preferred range is between about 450 C. and490 C.

Atmospheric pressure is preferred although higher pressures may be usedconsistent with flammability limits, in general, less than 100 psi.

Incorporated in the alkali metal hydroxide melt in accordance with myinvention is a soluble transition metal oxyanion co-catalyst which hasbeen found to cause an increase in the rate of the oxidativedehydrogenation reaction as well as an improvement in selectivity to thedesired olefin. The transition metal oxyanion is introduced into themelt as an alkali metal salt, having the formula [Q M O J wherein Q isan alkali metal, z is the valance of the transition metal oxyanion, M isthe transition metal which is selected from Group III-B thru VIII of thePeriodic Table, x and y are the number of atoms of M and 0 respectivelyin the anion, and w is a number from 1 to 6. The transition metaloxyanion co-catalyst thus has the formula M,,O,, wherein M, x, y, and zare as defined above. In the above formula, 2: has a value of 1 or 2, yhas a value of 3 to 7, and z has a value of 1 to 3. Any compoundsfitting this formula which are soluble in the alkali metal hydroxidemelt are suitable. The alkali 'but within the scope of the invention,are polymolybdates, polytungstates, and polyvanadates. 50

Preferably, the alkali metal salt of the above formula is dissolved inthe alkali metal hydroxide melt, and is present in a weight ratio rangeof from about 0.01% to 5% based upon the weight of the alkali metalhydroxide. It is most preferable that the compound be completelydissolved, and the solubility limits for each compound at varyingtemperatures are routinely determinable.

The gaseous hourly space velocity, i.e., the volumes of gaseous feed pervolume of molten sodium hydroxide per hour, is preferably from about 50to 800 and preferably from 100 to 600.

In the following examples runs were carried out utilizing a verticaltubular reactor composed of high purity alumina which measured about 40inches in length by 1.5 inches in outside diameter. The reactor isprovided with a concentric ceramic feed tube about A-inch outsidediameter which extended to the bottom of the reactor and was providedwith apertures at the bottom of the tube to provide a distribution meansfor the gaseous charge. The gaseous charge was passed downwardly throughthe feed tube and upwardly through a bed located in the annular spacebetween the feed tube and the inner wall of the reactor. The bedconsisted of a bottom layer of aluminum metal rings obtained by cuttingan aluminum tube about y -inch outside diameter by As-inch insidediameter to about %-ll'1Ch ring lengths. Above the rings there wasprovided a layer of tabular alumina (8-14 mesh). When activated aluminawas used, the aluminum rings were replaced by the activated alumina. Inthis reactor 100 ml. of molten caustic filled the space between therings and between the particles of packing and extended upwardly in thetube in the annular space. The co-catalyst is incorporated with thealuminum and the molten caustic. In some instances suflicient packingwas utilized so that the packing extended above the layer of the moltencaustic while in other runs smaller amounts of packing were used so thatthe molten caustic layer was above the top of the packing layer. Theoutside of the reactor tube was surrounded with three heaters so thatthe temperature of the reaction could be controlled uniformly to thedesired level throughout the reaction zone. The top of the reactor tubeis provided with conventional fittings to remove the reaction products.In the runs shown in the following examples a reaction temperature of490 C. was utilized in order to make comparisons of the other variables.Runs have been made in the broad temperature range and in the preferredtemperature range with the preferred and most preferred temperaturesgiving the best results.

It has been found that there is an induction period required to start upthe reaction. The fresh reactor assembly is pre-oxidized by passingoxygen through the assembly for several hours, generally overnight orabout 16 hours. Thereafter, the feed gas, consisting of the hydrocarbon,oxygen and nitrogen, is passed through the reactor for severaladditional hours before high conversions are attained.

In the following examples, runs are shown utilizing the apparatusdescribed. These runs illustrate specific embodiments of the inventionand show the preferred conditions for carrying out the reaction of thisinvention, and should not be construed as limiting the invention.

EXAMPLE I The following runs were made with 200 grams of sodiumhydroxide, 350 cubic centimeters of tabular alumina packing (4 x 8mesh), and grams of aluminum rings at a reaction temperature of 490 C.The feed gas composition is 40 mole percent propane, 12 mole percentoxygen, and 48 mole percent nitrogen. Runs A and B are comparative andnot within the present invention since no co-catalyst was used. Run C iswithin the invention since there is further incorporated in the sodiumhydroxide melt 0.1 weight percent of sodium metavanadate based on theweight of the sodium hydroxide. Run A was carried out at a gaseoushourly space velocity of 101 and a conversion of 12.1. The selectivityto propylene was 79.2 and the yield was 9.7. Run B was carried out at agaseous hourly space velocity of 102 with a conversion of 11.4,selectivity to propylene of 78.3, and a yield of 9.0. Run C was carriedout with the co-catalyst of the invention incorporated in the reactorunder otherwise the same conditions except that the gaseous hourly spacevelocity was adjusted so as to provide equivalent conversion level. InRun C the 4 gaseous hourly space velocity was 108 with a conversion of20.9, a selectivity to propylene of 79.8, and a yield of 16.6. Thus itcan be seen from a comparison of these runs that incorporation of 0.1weight percent sodium vanadate at approximately equal gaseous hourlyspace velocity results in much higher conversions at equalselectivities.

EXAMPLE II The conditions of Example I were repeated. Run D iscomparative, that is, without the incorporation of sodium vanadate andhad a gaseous hourly space velocity of 34 with a conversion of 30.9,selectivity of 63.0, and a yield of 19.5. Run E is within the limits ofthe invention and is the same as Example D except that 0.1 weightpercent sodium vanadate was incorporated in the melt. The gaseous hourlyspace velocity was 72 while maintaining a conversion of 30.4,approximately equivalent to the conversion in Run D. Selectivityincreased slightly to 65.0 and yield to 19.8. This example demonstratedthat at twice the throughput (space velocity), equivalent conversions,selectivities, and yields can be obtained using the co-catalyst of theinvention. That is, the use of this co-catalyst system essentiallydoubles the reaction rate.

While I have described my invention with great detail, variousmodifications, improvements, and variations should become readilyapparent without departing from the spirit and scope thereof.

I claim:

1. Oxidative dehydrogenation catalyst composition comprising a solutionof a compound having the formula (Q M O wherein Q is an alkali metal, Mis a transition metal selected from Group III-B to VIII of the PeriodicTable, x is 1 or 2, y is 3 to 7, w is 1 to 6 and z is 1 to 3 in moltenalkali metal hydroxide, wherein said compound constitutes from about0.01% to about 5% by weight of the composition.

2. The oxidative dehydrogenation catalyst composition of claim 1 whereinsaid compound is selected from the group consisting of sodiumdichromate, sodium molyb date, sodium tungstate, sodium permanganate,and sodium metavanadate.

3. The oxidative dehydrogenation catalyst composition of claim 2 whereinsaid compound is sodium metavanadate.

4. The oxidative dehydrogenation catalyst composition of claim 1 whereinsaid compound is sodium metavanadate and said alkali metal hydroxide issodium hydroxide.

5. The oxidative dehydrogenation catalyst composition of claim 1 whereinQ is sodium.

References Cited UNITED STATES PATENTS 3,692,860 9/1972 Boutry et al.260683.3 X

DANIEL E. WYMAN, Primary Examiner W. J. SHINE, Assistant Examiner U.S.Cl. X.R.

